| Message ID | 20160412115645.GA3730@vingu-laptop |
|---|---|
| State | New |
| Headers | show |
On 12 April 2016 at 23:09, Yuyang Du <yuyang.du@intel.com> wrote: > > Hi Vincent, > > On Tue, Apr 12, 2016 at 01:56:45PM +0200, Vincent Guittot wrote: > > Le Tuesday 12 Apr 2016 à 03:41:41 (+0800), Yuyang Du a écrit : > > > Hi Vincent, > > > > > > On Mon, Apr 11, 2016 at 11:08:04AM +0200, Vincent Guittot wrote: > > > > > @@ -2704,11 +2694,14 @@ static __always_inline int > > > > > __update_load_avg(u64 now, int cpu, struct sched_avg *sa, > > > > > unsigned long weight, int running, struct cfs_rq *cfs_rq) > > > > > { > > > > > - u64 delta, scaled_delta, periods; > > > > > - u32 contrib; > > > > > - unsigned int delta_w, scaled_delta_w, decayed = 0; > > > > > + u64 delta; > > > > > + u32 contrib, periods; > > > > > unsigned long scale_freq, scale_cpu; > > > > > > > > > > + /* > > > > > + * now rolls down to a period boundary > > > > > + */ > > > > > + now = now && (u64)(~0xFFFFF); > > > > > delta = now - sa->last_update_time; > > > > > /* > > > > > * This should only happen when time goes backwards, which it > > > > > @@ -2720,89 +2713,56 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, > > > > > } > > > > > > > > > > /* > > > > > - * Use 1024ns as the unit of measurement since it's a reasonable > > > > > - * approximation of 1us and fast to compute. > > > > > + * Use 1024*1024ns as an approximation of 1ms period, pretty close. > > > > > */ > > > > > - delta >>= 10; > > > > > - if (!delta) > > > > > + periods = delta >> 20; > > > > > + if (!periods) > > > > > return 0; > > > > > sa->last_update_time = now; > > > > > > > > The optimization looks quite interesting but I see one potential issue > > > > with migration as we will lose the part of the ongoing period that is > > > > now not saved anymore. This lost part can be quite significant for a > > > > short task that ping pongs between CPUs. > > > > > > Yes, basically, it is we lose precision (~1ms scale in contrast with ~1us scale). > > > > But with a HZ set to 1000 and a sched-slice in the same range, having a precision > > of 1ms instead of 1us makes the precision of load tracking of short task quite > > random IMHO. > > > > you can keep recording this partial period without using it in the load tracking. > > Something like below keep precision without sacrifying the optimization. > > The residue is accumulated and rolled over to next update every time. But its > state is runnable/not-runnable, or running/not-running? yes, this need to be sorted > > > > --- > > kernel/sched/fair.c | 16 ++++++++++++---- > > 1 file changed, 12 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 68273e8..b234169 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -674,6 +674,12 @@ void init_entity_runnable_average(struct sched_entity *se) > > struct sched_avg *sa = &se->avg; > > > > sa->last_update_time = 0; > > + /* > > + * sched_avg's period_contrib should be strictly less then 1024 * 1024, so > > + * we give it 1023 * 1024 to make sure it is almost a period (1024us), and > > + * will definitely be updated (after enqueue). > > + */ > > + sa->period_contrib = 1023*1024; > > sa->load_avg = scale_load_down(se->load.weight); > > sa->load_sum = sa->load_avg * LOAD_AVG_MAX; > > /* > > @@ -2698,10 +2704,6 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, > > u32 contrib, periods; > > unsigned long scale_freq, scale_cpu; > > > > - /* > > - * now rolls down to a period boundary > > - */ > > - now = now && (u64)(~0xFFFFF); > > delta = now - sa->last_update_time; > > /* > > * This should only happen when time goes backwards, which it > > @@ -2712,6 +2714,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, > > return 0; > > } > > > > + /* Add how much left for the current period */ > > + delta += sa->period_contrib; > > + > > /* > > * Use 1024*1024ns as an approximation of 1ms period, pretty close. > > */ > > @@ -2720,6 +2725,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, > > return 0; > > sa->last_update_time = now; > > > > + /* Get how much left for the next period */ > > + sa->period_contrib = delta & (u64)(0xFFFFF); > > + > > scale_freq = arch_scale_freq_capacity(NULL, cpu); > > scale_cpu = arch_scale_cpu_capacity(NULL, cpu); > > > > > But as I wrote, we may either lose a sub-1ms, or gain a sub-1ms, statistically, > > > they should even out, given the load/util updates are quite a large number of > > > samples, and we do want a lot of samples for the metrics, this is the point of > > > the entire average thing. Plus, as you also said, the incomplete current period > > > also plays a (somewhat) negative role here. > > > > > > In addition, excluding the flat hierarchical util patch, we gain quite some > > > efficiency.
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 68273e8..b234169 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -674,6 +674,12 @@ void init_entity_runnable_average(struct sched_entity *se) struct sched_avg *sa = &se->avg; sa->last_update_time = 0; + /* + * sched_avg's period_contrib should be strictly less then 1024 * 1024, so + * we give it 1023 * 1024 to make sure it is almost a period (1024us), and + * will definitely be updated (after enqueue). + */ + sa->period_contrib = 1023*1024; sa->load_avg = scale_load_down(se->load.weight); sa->load_sum = sa->load_avg * LOAD_AVG_MAX; /* @@ -2698,10 +2704,6 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, u32 contrib, periods; unsigned long scale_freq, scale_cpu; - /* - * now rolls down to a period boundary - */ - now = now && (u64)(~0xFFFFF); delta = now - sa->last_update_time; /* * This should only happen when time goes backwards, which it @@ -2712,6 +2714,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, return 0; } + /* Add how much left for the current period */ + delta += sa->period_contrib; + /* * Use 1024*1024ns as an approximation of 1ms period, pretty close. */ @@ -2720,6 +2725,9 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa, return 0; sa->last_update_time = now; + /* Get how much left for the next period */ + sa->period_contrib = delta & (u64)(0xFFFFF); + scale_freq = arch_scale_freq_capacity(NULL, cpu); scale_cpu = arch_scale_cpu_capacity(NULL, cpu);